1. Field of the Invention
The present invention relates to image forming devices and, more particularly, to single pass color printers.
2. Description of the Related Art
In an electrophotographic process, such as xerography, an optical device scans a light pattern along a charged photosensitive surface to form a latent image corresponding to an electrical or optical input. The resulting pattern of charged and discharged areas on the surface forms an electrostatic latent image corresponding to the original image. Developing devices of the electrostatic copying machine develop the latent image using either yellow, magenta, cyan, and/or black developing toners for subtractive color systems or red, blue and green developing toners for additive color systems. The developing toners are composed of electrostatically attractable powder, which are attracted to the latent image areas formed on the charged photosensitive surface. The developed image is then transferred to a predetermined image medium, e.g., paper, to produce a reproduction and a permanent record of the original image.
In single-pass color printers, the red, green and blue color separation latent images are superimposed on an electroreceptor belt or drum prior to transfer to the recording medium. The color separation latent images are superimposed on the electroreceptor belt or drum by successive imaging stations located adjacent the electroreceptor belt or drum. Each imaging station records a color separation latent image corresponding to one of the colors. The single color separation latent image is developed with toner particles of a color complementary thereto prior to transfer to the recording medium. Various recording media can be used in conjunction with these color printers such as a sheet of paper, a transparency, etc. Various types of single-pass printers are known in xerographic art, as described below.
U.S. Pat. No. 5,113,202 to Loce et al. discloses a single pass highlight color printing system utilizing the disparity between the on-time of a printing system and the on-time of an optical image bar. The image bar output is adapted to be separated into two image outputs which expose two separate areas of a photoreceptor. Each exposed area is developed by a toner of a desired color and the developed image, having two colors thereon, is transferred to a copy sheet to form a two-color output copy.
U.S. Pat. No. 4,403,848 to Snelling et al. discloses a multi-color printer using an additive color process to provide either partial or full color copies. Multiple scanning beams, each modulated in accordance with distinct color image signals, are scanned across a photoreceptor at relatively widely separated points, and buffer means are provided to control the timing of the different color image signals to assure registration of the color images with one another. Each color image is developed prior to scanning of the photoreceptor by the next succeeding beam. Following the development of the last color image, the composite color image is transferred to a copy sheet. In an alternate embodiment, an input section for scanning color originals is provided. The input section outputs color image signals for use by the printing section to make full color copies of the original.
U.S. Pat. No. 4,791,452 to Kasai et al. discloses single-colored image printing and multi-colored image printing being carried out by an image forming apparatus in response to color signals. The image forming apparatus includes an image carrier, a first image forming unit having a first developer wherein a first color developing agent is stored, and a second image forming unit having a second developer wherein a second color developing agent is stored. A single color image is formed on the image carrier when only a first color signal is received by the apparatus in the single-colored image printing. When a single-colored image printing is carried out by the first developer, a prescribed bias voltage is applied to the second developer to protect the second color developing agent from the contamination by the first color developing agent of the single color image formed on the image carrier.
U.S. Pat. No. 4,416,533 to Tokunaga et al. discloses a nonimpact printer. Electric charges are distributed over the surface of an electrophotographic photosensitive drum of the printer by a corona charger, and the charged surface of the drum is exposed to a first information light. The induced latent image is developed by toner of a first color and the charged surface of the drum with the developed image of the first color, is again exposed to a second information light. The second induced latent image is developed by toner of a second color, so that a bicolor toner image is formed on the surface of the drum. The bicolor toner image is transferred onto a printing medium to obtain a print.
U.S. Pat. No. 4,578,331 to Ikeda et al. discloses an electrophotographic color image forming process wherein three light beams, each representing a color image information of a color document to be recorded obtained by color separation, are projected against an electrophotographic photosensitive member to form electrostatic latent images. The latent images are developed by toners of the three different colors and printed by transfer-printing to record a color image. The color image information is simultaneously written to a surface of the photosensitive member, either by successively writing a plurality of sets of three scanning lines, each representing image information of one color, or by writing image information of different colors of the same set separately in three different zones. The scanning lines represent image information of different colors for forming a repeating series of three strips of different colors. The electrostatic latent images formed on the scanning lines are excited in positions immediately before developing sections of respective colors and developed by the toners of respective colors to produce toner images of different colors, which are printed by transfer-printing on a transfer-printing sheet.
To achieve a quality image with the above-described single-pass printers, which image is not blurred in appearance and which does not contain unwanted artifacts, the registration of color separation latent images must be accurately provided in superimposed relationship while avoiding any motion induced image degradation. Accordingly, the motion of the electroreceptor belt or drum must be finely controlled, particularly in the span of the electroreceptor belt or drum which encompasses the imaging and developing stations forming the images. The following U.S. Pat. Nos. 5,101,232 and 5,153,644 describe two known methods to control the velocity of the photoreceptor and to correct image distribution due to vibrational and speed distortions.
U.S. Pat. No. 5,101,232 to Evans et al. discloses an apparatus and associated method for controlling the velocity of the photoreceptor within a reprographic machine having a seamed, web type photoreceptor. The images are separated by unexposed interdocument regions on the photoreceptor. The reprographic machine further includes a registration apparatus for registering copy substrates with developed latent images. The process of assuring that the seamed region of the photoreceptor lies within an interdocument region begins by first sensing an actual phase relationship between the photoreceptor seam and activity of the registration apparatus and then calculating a phase error value by comparing the actual phase relationship to a desired phase relationship. Next, the system determines an adjustment photoreceptor velocity as a function of the phase error. Subsequently, the photoreceptor is moved at a fixed velocity during exposure of the images. Changing the calculated reference and hence photoreceptor velocity is restricted to the interdocument zone, so that there are no velocity changes except when the interdocument zone is passing through the imaging station. This ensures that the registration requirements and image quality specifications are simultaneously accomplished.
U.S. Pat. No. 5,153,644 to Yang et al. discloses correction of image distortion in a reproduction machine due to vibrational or speed distortions in or between a moving photoreceptor and an imaging device projecting raster lines on the photoreceptor as it moves past the imaging device. An encoder senses the speed of the photoreceptor and also the relative vibrational motion between the photoreceptor and the imaging device and provides electrical signals to a signal separator for separating the electrical signals into lower and higher frequency signals. Further, a servo motor drives the photoreceptor and compensates for the lower frequency increases or decreases in the speed of the photoreceptor. The servo motor is driven by the separated lower frequency signals. An optical system pivots the projected raster lines to compensate for the image distortions which are of the higher frequencies. The optical system is driven by the higher frequency signals from the signal separator.
However, all of the above patents fail to disclose or teach accurate registration of superimposed color separation latent images by controlling the velocity of the span of the electroreceptor belt or drum which encompasses the imaging and developing stations forming the color separation latent images.